The present invention relates to a process for processing a signal for side-looking and synthetic aperture radar systems and to a circuit for performing this process. It is used in remote detection, cartography by radar, etc and more specifically in the observation of the state of the ocean, where it permits the real time determination of the directional spectrum of the swell.
The technique of side-looking, synthetic aperture radar is used in active remote detection for obtaining a cartography of an extremely high frequency signal transmitted back by a surface. A very good spatial resolution can be obtained by a coherent demodulation of the signal received, followed by matched filtering of the signal. The thus obtained cartography is called the radar image. A description of this technique appears in numerous works or articles, including the following:
(1) R. O. HARGER "Synthetic Aperture Radar Systems" New York, Academic Press, 1970 PA1 (2) A. W. RIHACZEK "Principles of High Resolution Radar" New York, MacGraw Hill, 1969 PA1 (3) K. TOMIYASU "Tutorial Review of Synthetic-Aperture Radar (SAR) with Applications to Imaging of the Ocean Surface" Proceedings of the IEEE, Vol. 66, No. 5, May 1978, pp. 563-583. PA1 (4) W. J. VAN DE LINDT "Digital Technique for Generating Synthetic Aperture Radar Images", IBM J. Res. Develop. September 1977, pp. 415-432 PA1 (5) HOMER JENSEN et al "Cartographie par Radar" Pour la Science, December 1977, pp. 80-92.
The image produced by a synthetic aperture radar carried on board a satellite and directed towards the ocean surface makes it possible to estimate the surface state thereof. This estimate is provided by calculating the bidimensional Fourier transform of the radar image, which makes it possible to determine the directional spectrum of the swell. The thus obtained information can be used in monitoring the state of the ocean (detecting storms, navigation aids) or in oceanographic research. The specifications of such a system impose numerous restraints, namely overall coverage, all-weather capacity, periodicity of a few hours and very rapid transmission of the information. These constraints can only be satisfied by a radar system carried by a satellite and associated with a system able to rapidly calculate the bidimensional Fourier transform of the radar image.
The presently used processes for calculating the Fourier transform of a radar image take place in two stages, i.e. the generation of the image and the calculation of the Fourier transform thereof.
The image can be generated by optical or digital process methods. The image is obtained on a photographic support in the first case and in the form of an image digitized on the magnetic tape of a data processor in the second case. The Fourier transform of the radar image is then determined by optical processing for photographs or by a computer for the digitized images.
However, these known processes are not well suited to the real time determination on board a satellite of the Fourier transform of the radar image. Thus, even if pulse compression forming the first stage in image generation can be carried out in real time by hybrid methods using acoustic surface wave devices, the second acoustic aperture stage requires very long processing times which are incompatible with a real time processing constraint. Moreover, image generation must be completed before it is possible to start the calculation of the Fourier transform. It is difficult to use optical processing methods in an automatic manner on board a satellite, due to the equipment and personnel required for performing the same.
The object of the present invention is to reduce the complexity and duration of processing required for obtaining, directly from the signal received by the radar and after coherent demodulation and pulse compression, the Fourier transform in the azimuth of the radar image. Naturally, it is then always possible to obtain an estimate of the radar image by reverse Fourier transformation.
In order to give a better idea of the questions involved, it is worth briefly referring to the diagrammatic structure of a side-looking, synthetic aperture radar system, as illustrated in FIG. 1. The structural details and the functions of the different organs shown can be gathered from the references indicated hereinbefore.
As illustrated, this system comprises an extremely high frequency wave generator 10 associated with a clock 12 fixing the repetition frequency Fr of the transmission, means 14 for modulating the frequency of the wave transmitted by generator 10, an amplifier 16, a circulator 18 and an antenna 20. This subassembly corresponds to the transmission means of the system.
The system shown also comprises a low noise amplifier 22, a pulse compression circuit 24 and a coherent detection circuit 26, which is also connected to generator 10. This detection takes place on the components of the signal in phase and in phase quadrature. This subassembly corresponds to the reception means of the system.
The system shown also comprises delay networks 30/1, 30/2 . . . 30/p connected to the transmission clock 12 and gates 32/1, 32/2 . . . 32/p able to select samples s.sup.1, s.sup.2 . . . s.sup.p located in the corresponding gates from the signal supplied by the detection circuit. The overall circuit able to supply these samples carries the reference numeral 33.
Finally, the system comprises means 34 for forming the radar image from the signals s.sup.1, s.sup.2 . . . s.sup.p, as well as means 36 for calculating from the said image the Fourier transform thereof.
This equipment is generally carried by a moving craft (satellite, aircraft, etc.) in such a way that transmission means 40 are provided thereon for transmitting the desired data to the ground. Said data can comprise Fourier components obtained at the output of circuit 36 (connection 42 between 36 and 40) or by the image produced by means 34, in which case the output of said means is directly connected to the transmission means 40 (connection 44) and the Fourier transformer is located on the ground. The data can also comprise signals sampled further upstream, such as e.g. at the output of the coherent detection circuit 26 and of the transmission clock 12 (connection 46).
The operating principle of this system is based directly on that of synthetic aperture radar systems. Radar transmission is discontinuous and is performed at the repetition frequency F.sub.r. Frequency modulation and then pulse compression make it possible to improve the resolution in the radial direction. Coherent detection permits the aperture synthesis by which the range of the lateral antenna can be reduced. The gates are opened with a certain delay compared with the transmission times of the radar wave and the signals passing through the same correspond to echos coming from obstacles located at predetermined distances from the antenna. Thus, these signals are samples associated with the different spacing gates. The image is formed from these samples by optical or electronic means, in the manner indicated hereinbefore.